Machine I INTRODUCTION Machine, simple device that affects the force, or effort, needed to do a certain amount of work. Machines can make a tough job seem easier by enabling a person to apply less force or to apply force in a direction that is easier to manipulate. Machines lessen the force needed to perform work by lengthening the distance over which the force is applied. Although less force is subsequently used, the amount of work that results remains the same. Machines can also increase the speed at which work makes an object travel, but increasing speed requires the application of more effort. There are four types of simple machines: the lever, the pulley, the inclined plane, and the wheel and axle. Each machine affects the direction or the amount of effort needed to do work. Most mechanical machines, such as automobiles or power tools, are complex machines composed of many parts. However, no matter how complicated a machine is, it is composed of some combination of the four simple machines. Although these simple machines have been known and used for thousands of years, no other simple machines have been discovered. Two other common simple machines, the screw and the wedge, are really adaptations of the inclined plane. Some common examples of simple machines are the shovel (a form of lever), the pulley at the top of a flagpole, the steering wheel of an automobile (a form of wheel and axle), and the wheelchair ramp (a form of inclined plane). An everyday example of a complex machine is the can opener, which combines a lever (the hinged handle), a wheel and axle (the turning knob), and a wedge (the sharpened cutting disk). II WORK Machines help people do work by changing the amount of force and the distance needed to move objects. Work, in physics, is the amount of force used to move an object multiplied by the distance over which the force is applied. This can be written in mathematical terms: Work = Force × Distance Force is defined as a push or a pull exerted on one body by another, such as a hand pushing a book across a table. Distance refers to the distance a load is moved by the force. The advantage that a machine gives its user by affecting the amount of force needed is called the machine's mechanical advantage, or MA. Knowing the mechanical advantage of a machine allows a user to predict how much force is needed to lift a given object. A How Machines Work A machine can make a given task seem easier by reducing the amount of force needed to move an object, by changing the direction in which the force must be applied, or by doing both. A machine decreases the amount of force needed by increasing the distance over which the effort is applied to move the object. The amount of work needed to overcome gravity and lift a given load always remains the same, but spreading the necessary effort out over a longer distance makes the task seem easier. This is why walking gradually up a gentle slope is easier than walking up a steep slope. The distance walked on the gentle slope is longer, but the effort needed to reach the top is less. A gentle slope is a form of inclined plane. Applying effort over a greater distance takes more time, and this slows down the speed of work. Some machines can actually speed up a task. They do this by reducing the distance over which the effort is applied. If the distance in the equation defining work (Work = Force × Distance) is reduced, then the force must therefore be increased to keep work constant. Increasing the speed at which a task is performed requires more force than would otherwise be necessary. The wheel and axle and certain types of levers are simple machines that can either speed up a task (requiring more effort) or slow down a task (requiring less effort). The various gears on a multispeed bicycle (another complex machine) work in a manner similar to that of the wheel and axle. Some gears require more effort, but they make the bicycle travel faster on flat terrain. Other gears require less effort and are useful for climbing hills. People use simple machines, such as levers and pulleys, to make manual chores easier. The mechanical energy in a person's muscles makes the machine do work. Not all machines use muscle power, however, to do work. A complex machine, such as an airplane engine or an elevator, is made up of many simple machines. Airplane engines and elevators are not powered by hand. Complex machines often use the energy stored in chemical substances, such as airplane fuel or the energy stored in electricity, to provide the necessary force to do work. An airplane engine uses the combustion, or rapid burning, of airplane fuel to power the engine that turns the propeller. An elevator uses large engines, usually powered by electricity, to pull cables that raise and lower the elevator car. Electricity also powers the levers that help open and shut the elevator doors. B Mechanical Advantage and Friction Measuring the mechanical advantage (MA) is a mathematical way to determine how much a machine affects the amount of force needed to do work. Scientists find the mechanical advantage of a machine by dividing the force the machine delivers by the effort put into the machine. The theoretical, or ideal, mechanical advantage of a machine is the advantage it would produce if the machine were perfect. In simple machines, the main source of imperfection is friction. Friction results from two bodies moving against each other in different directions. Friction always opposes motion and makes doing work harder. Since friction is present in almost every machine, the actual mechanical advantage is always less than the theoretical mechanical advantage. Because simple machines increase mechanical advantage by increasing the distance over which the effort is applied, one way to compute theoretical mechanical advantage is to divide the distance the effort is applied by the distance the load actually travels. For example, raising a load 5 m (16 ft) off the ground is easier if the load is moved up a gradual slope, or an inclined plane, rather than lifted straight up. Moving the load along a 10-m (32-ft) inclined plane would provide a mechanical advantage of 10 divided by 5, or 2. This means that the work was twice as easy, or that only half as much effort was needed to raise the load. Because of the inclined plane, however, the load needed to be pushed twice as far to end up 5 meters above the ground. C Efficiency Another factor that people sometimes compute for machines is their efficiency, or the ratio of the work that results to the amount of work put into the machine. The efficiency of a machine is usually expressed as a percentage and can vary from 5 percent to 95 percent. A perfect machine would be 100 percent efficient. Most simple machines are very efficient, but they always lose some efficiency due to friction. An automobile engine is much less efficient because much of the energy used to move the crankshaft is lost to friction in the form of heat dissipating from the engine. III TYPES OF SIMPLE MACHINES The four simple machines each function in different ways, but they all change the direction or the amount of effort put into them. All four of these machines can be used to decrease the amount of force needed to do work or to change the direction of the force. The wheel and axle and some levers can also be used to increase the speed of performance of a task, but doing so always increases the amount of force needed. A Inclined Plane Nuts Nuts are pieces of metal with a hole in the middle. They are screwed on the end of a bolt as a fastening. Nuts come in varying shapes, depending on their intended use. © Microsoft Corporation. All Rights Reserved. Ramps and staircases are simple examples of inclined planes. An inclined plane is an object that decreases the effort needed to lift an object by increasing the distance over which the effort is applied. This increase in distance allows a person to move a large object to a certain height while applying less force than would otherwise be needed. (Without the plane, a person would need to lift with a force equal to the entire weight of the object.) The tradeoff is that with the inclined plane, the person must move the object a farther distance. An inclined plane also changes the direction--from straight up to along the angle of the plane--of the effort applied. The amount of work done is the same whether the person lifts the object straight up or along an inclined plane. The MA of an inclined plane equals the length of the plane divided by the height to which the object is raised. A long inclined plane at a small angle has a greater mechanical advantage than a steep inclined plane, because the effort is applied over a greater distance. A wedge is a double inclined plane, with a plane on each side. Wedges are often used to split wood, changing the downward direction of the force from a sledgehammer to a sideways force toward the wood being split. A screw is a form of inclined plane in which the plane is wrapped around an axis, or pole. The MA of a screw is related to the pitch of the threads (the distance along the axis of the screw from one thread to the next) and the diameter of the axis. There are two different types of screws: fastening screws and lifting screws. Fastening screws are used to join things together. Examples of fastening screws are wood or metal screws, which have threads that dig into the materials being joined. The materials are held together by a combination of friction on the threads and compression of the screw by the materials. Other screws, sometimes called machine screws or bolts, have threads that are matched by the threads on the inside of a nut. Lifting screws are used to lift loads or to exert forces on other bodies. An example of a lifting screw is the screw jack used to change tires on a car. Lifting screws are usually lubricated to reduce friction, but some friction with lifting screws is helpful so that the screw can safely hold the load. B Lever One of the most commonly used simple machines is the lever. A seesaw is an example of a lever. The human arm is actually a lever, and the muscles apply the force needed to lift weight or move objects. A lever consists of a bar that rotates around a pivot point, which is called the fulcrum. The force applied by the user is the effort. The object being lifted is called the load. There are three classes of levers, which vary in the placement of the effort, the load, and the fulcrum along the bar. In a Class 1 lever, the fulcrum lies between the effort and the load, as in a seesaw. In a Class 2 lever, the fulcrum lies at one end, the effort is applied at the other end, and the load is in the middle, as in a wheelbarrow. In a Class 3 lever, the fulcrum is again at one end, but the load is at the other end, and the effort is applied in the middle. The human forearm is a Class 3 lever. The elbow is the fulcrum, and the forearm muscles apply the effort between the elbow and hand. Tweezers are another example of a Class 3 lever. One of the limitations of levers is that they only operate through relatively small angles. The MA of a lever is the distance from the fulcrum to the point where the force is applied divided by the distance from the fulcrum to the load. The MA is maximized when the load is close to the fulcrum and the effort is far from the fulcrum. In this case, a small effort can move a large load. C Pulley The pulley is a special type of wheel, called a sheave, which has a groove cut into the edge to guide a rope, cable, or chain. Pulleys are used at the top of flagpoles and in some types of window blinds. If a single pulley is used, the mechanical advantage is 1, and the only advantage of using the pulley is that the direction of the force needed is changed. For example, to raise window blinds, a downward pull on a cord is required. When multiple pulleys are combined (in what is called a block and tackle), they can have mechanical advantages greater than 1, because they increase the distance the rope travels, thereby increasing the distance over which the effort is applied. The MA of a block and tackle is equal to the number of strands of rope on the part of the block and tackle that is attached to the load. Using a combination of pulleys that results in three strands of rope attached to the load requires the user to pull the rope three times farther than the load actually moves. This results in an MA of 3, which means that one-third as much effort is required to move the load. The rope on a pulley causes a good deal of friction, and this limits the number of pulleys that can be used. D Wheel and Axle The wheel and axle is similar in appearance to a pulley, with one major difference: the wheel is fixed to the axle, as is the steering wheel of a car. A user applies effort to the large outer wheel of the steering wheel to move the load at the axle. The MA of a wheel and axle is equal to the radius of the wheel divided by the radius of the axle. The radius of the wheel, and therefore its circumference, is usually much larger than the radius of the axle. Therefore, the distance over which the effort is applied is much greater than the distance the load, which is placed at the axle, moves. The difference in the sizes of the wheel and axle can result in a large mechanical advantage. Some common examples of a wheel and axle are a doorknob and a round water faucet handle. IV COMPLEX MACHINES Many everyday objects are really combinations of simple machines. Such combinations are known as complex machines. The doorknob is a wheel and axle system that transfers the force applied by a person to a system of levers. The levers move the bolt and unlatch the door. A pair of pliers is really two Class 1 levers with the same fulcrum (the pivot pin). Pliers usually have a mechanical advantage of 5 or higher. A pair of scissors is a pair of pliers with wedges as the cutting edge. Cutting something thick or hard is easier when the scissors are opened wide and the object is placed near the pivot pin. This placement decreases the distance between the load and the fulcrum, giving the scissors a higher MA than if the cutting was done near the tip of the scissors. Some complex machines are very complicated. An automobile is one such machine. The engine contains many levers, wheels and axles, and pulleys. The whole engine is held together by threaded bolts, which are a form of inclined plane. The transmission uses gears, which are a form of wheel and axle with specially shaped teeth on the outside of the wheels. Two gears fit together and transfer force and power from one gear shaft to another. By choosing the size of the gears, the speed and direction of the rotation of the axles can be controlled. Even devices that do not seem to be mechanical use simple machines. A computer, which is thought of as an electronic device, has a cooling fan. This fan is a complex machine in which the motor shaft turns the fan, which is a form of wheel. The disk drive uses a wheel and axle to turn the disk and a system of levers to position the heads that read and write the data on the disk. V HISTORY The history of machines dates back thousands of years. Although the date of the first use of simple machines is not known, the lever is believed to be the first simple machine that was utilized by humans. However, someone choosing a long, gradual approach up a mountain rather than walking up a steeper, shorter path would have been taking advantage of an inclined plane. The first levers were probably branches or logs used to lift heavy objects. People used a counterbalanced lever called a shadoof in ancient Egypt for lifting irrigation water. People also used such a device for lifting soldiers over battlements. Metal or stone wedges have been used since ancient times for splitting wood. People used wooden wedges to split rocks by placing dry wooden wedges into cracks in rocks and then allowing the wedges to swell by absorbing water. Historians believe the people of ancient Mesopotamia (an early civilization near modern-day Iraq) used wheels as early as 3500 Minor used spoked wheels, which were lighter than solid wheels, as early as 2000 BC. The Greek inventor Archimedes (287-212 BC) BC. Chariots in Asia developed a screw-type device known as Archimedes' screw for raising water. Some modern water pumps still use this principle. According to legend, Archimedes also used a block and tackle to pull ships onto dry land. Machines can transform natural energy, such as wind and falling water, into work. Waterwheels, first used in ancient Greece and Rome, and later adopted by Europeans in the 12th century, used the water falling from a waterfall to turn large wheels (see Waterpower). The windmill also uses the same wheel and axle principle to magnify and change the direction of force to do work. Grinding wheels connected to waterwheels can grind grain for making flour or power large saws for sawing wood. Pumps connected to windmills transform the rotary motion of a windmill into reciprocating (back and forth) motion, which is used to pump water from the ground. Waterwheels and windmills can also be connected to electrical generators to produce electricity. Complicated machines such as the power loom (patented in 1786) helped cultivate the improvements seen in Great Britain during the first Industrial Revolution at the end of the 18th century. Later Industrial Revolutions elsewhere brought about the invention of even more complex machines, such as the cotton gin (used to separate cotton fibers from seeds), the mechanical reaper (used to cut grain), and the automobile. Contributed By: Odis Hayden Griffin, Jr. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
